Scientists had never seen such material with learning capabilities before!

A material capable of memorizing stimulation external conditions it has undergone and to adapt to them, just like our brain : difficult to imagine that this type of material exists, and yet, vanadium dioxide (VO₂) is the very first representative! This semiconducting metal oxide was already known to possess an insulator-to-metal transition, making it perfect for uses in the field of electronics, or even other, more amazing ones, such as smart windows ! But researchers have discovered other surprising properties, described in a study published in Nature.

Originally, vanadium dioxide has an abrupt transition from insulator to conductor at 68°C: a mixture between a Mott transition and an Peierls transition. When the temperature is below 68°C, the electrons of VO₂ are as if “attached” to atoms, because of the Coulomb repulsion between two atomic sites. But’energy brought by the heating of the material allows the electrons to leave their site and to move in the VO2.

The Peierls transition, on the other hand, concerns the structure of the crystal lattice. The latter is distorted below the transition temperature, creating a gap of energy between conduction band and the valence band : the electrons are not free to move in the material. But when the temperature increases, the distortion disappears, allowing the electrons to leave their atomic sites.

A discovery by chance

Already surprising for this insulator-to-metal transition, vanadium dioxide is the subject of numerous studies, in order to determine in which fields it could be used. This is what Mohammad Samizadeh Nikoo, a doctoral student at the Swiss Federal Institute of Technology in Lausanne (EPFL), did. Its initial goal was to study his so-called “volatile” memory, that is to say that “the material returns to the insulating state just after the removal of the excitation”specifies Samizadeh Nikoo in a EPFL press release, first author of the study. For this, he sent many times a Electric power in a vanadium oxide sample. “An electrical impulse passes through the material and continues on its way to the exit”says the scientist.

The pulse makes it possible to heat the material, and therefore, to perform the expected insulator-metal transition. Once the current has passed, the cooling involves a return to the initial insulating state. The researcher wanted to know how long this return to the insulating state took. But that’s where he noticed another amazing effect: the vanadium dioxide behaved differently during the second electrical impulsethe time to return to the insulator state was not the same as for the first pulse.

It was as if its structure had adapted to it. “He remembers his last transition and anticipates the next one. This is an unexpected memory effect. This is not related to electronic states, but to the structure physical of the material. It is a novelty from a scientific point of view, because no other material behaves like this”, says Elison Matioli, co-author of the study and director of the laboratory in which the measurements were taken. Currently, researchers have shown that this memory could last up to three hours. “If he remembers it this long afterwards, it is possible to say that this memory can last several days, but currently we do not have the measurement tools necessary to prove it”continues E.Matioli.

One thing is certain: this discovery opens the gate to many apps potential, especially for calculation operations which require memory. Indeed, thanks to its strange behavior, vanadium dioxide is able to store more data (of different states) than conventional materials which are limited to information binaries. In his case, we speak of structural memory, where the information is stored directly in the structural modifications he has made, while for other materials, we speak of electronic memory, where only two electronic states are possible.